1. Field of the Invention
The present invention relates to data communications. In particular, this invention relates to data communication transceivers that can be trained, and to methods for training such data communication transceivers.
2. Description of the Related Art
Current data communication transmitter-receivers (“transceivers”), such as pulse code modulation (“PCM”)-based modems and digital subscriber line (“xDSL”) modems, have two separate modes in which they may operate: a training mode and a data mode. When a data communication session commences between a pair of transceivers coupled by a communication channel, the transceivers initially operate in a training mode in which the receiver of each data communication transceiver is trained using predetermined signals. Receiver training in the training mode may include automatic gain control (“AGC”), timing recovery, echo cancellation, equalization, and channel learning training, and initial training for other adaptive algorithms. The training process attempts to adjust these functions for best performance in the communication environment. After training has been completed, the data communication transceivers exit the training mode and enter the data mode. In the data mode, user data transmitted by one data communication transceiver to the other is transmitted at a fixed rate that in the previous training mode was determined to be optimum. Adaptive algorithms may update in the data mode in response to changes in the communication environment. However, if communication is not stable in the data mode, for instance if the bit error rate is higher than expected, then the transceivers will go back to the training mode and retrain. After retraining, the transceivers return to the data mode, and further user data transmitted by one data communication transceiver to the other is transmitted at a fixed rate consistent with conditions existing in the communication channel during the retraining period.
Current data communication transceivers communicate (i.e., transmit and/or receive) user data only in the data mode, and do not communicate any user data while training or retraining in the training mode. In current modems such as PCM and xDSL modems, each training or retraining session can take a substantial amount of time, for instance 10-20 seconds. Communication channel conditions may require repeated retraining sessions during the course of a communication session. Retraining effectively disconnects and reconnects the transceiver, and many applications such as Internet audio, video, telephony, and multimedia applications suffer if communication sessions are subject to interruption such as occurs during retraining. These drawbacks result from the above-described method of operation of current data communication transceivers, which may be termed “training-then-working.”